1. Field of the Invention
The present invention relates to a semiconductor integrated circuit that has a plurality of chips on a lead frame.
2. Description of Related Art
In a metal oxide semiconductor field effect transistor (MOSFET), there are a P-channel MOSFET and a N-channel MOSFET. The N-channel MOSFET has a low ON-resistance in comparison with the P-channel MOSFET. Accordingly, the N-channel MOSFET is used for a particular use that requires flowing a large electrical current, such as for driving a motor. JP-A-2000-307397 discloses a high-side switch that uses the N-channel MOSFET.
As shown in FIGS. 8, 9, an integrated circuit (IC) 1 for driving a motor has lead wires 2, 3 for connecting to a power supply, a lead wire 4 for inputting a control signal, and lead wires 5, 6 for connecting to the motor. The IC 1 also has a N-channel MOSFET 7, a diode 8, a driving IC 9, and capacitors 10, 11, 12, 13 on a lead frame. A drain and a source of the N-channel MOSFET 7 are connected between the lead wires 2, 5. A cathode and an anode of the diode 8 are connected between the lead wires 5, 6. The diode 8 is a fast recovery diode. The driving IC 9 produces a PWM driving signal for a gate of the MOSFET 7 based on the control signal.
The undersurface side of the MOSFET 7 is the drain, and the undersurface side of the diode 8 is the cathode. Accordingly, the MOSFET 7 and the diode 8 cannot be mounted on a single island due to a difference between their electrical potentials. As a result, the MOSFET 7, the diode 8, and the driving IC 9 are mounted on separate islands 14, 15, 16, respectively.
When electrical currents flow through the MOSFET 7 and the diode 8, the MOSFET 7 and the diode 8 generate heat. The amounts of the heat from the MOSFET 7 and the diode 8 vary according to a specification of the motor and a duty ratio of the PWM driving signal. Accordingly, it is necessary to have adequate sizes of the islands and the lead wires so that the heat generated from the MOSFET 7 and the diode 8 can be radiated from the islands 14, 15 via inner lead wires and outer lead wires. As a result, it is difficult to reduce a package 17 of the IC 1 in size.
In such an IC 1, even when non-chip-mounted surfaces of the islands 14, 15 are exposed from the undersurface of the package 17 and a cooling plate is mounted to the surfaces of the islands 14, 15, it is necessary to have an insulating sheet between the islands 14, 15 and the cooling plate because the islands 14, 15 have complicated structures and it is difficult to isolate the islands 14, 15. As a result, thermal resistances are increased, and cooling capabilities are decreased.